The present invention generally relates to a developing material for use in electrography and a developing method utilizing said developing material, and more particularly, to a developing material in powder form for developing electrostatic latent images including electrically insulative toner particles and carrier particles, and a developing method for developing the electrostatic latent images employing said developing material.
Conventionally, there have been widely employed for actual applications, electrophotographic copying apparatuses which utilize two or dual-component developing materials such as the developing material for cascade development including carrier particles, for example, a glass beads and the like and electrically insulative toner particles, or the developing material for magnetic brush development composed of carrier particles of iron particles and the like and electrically insulative toner particles, etc. In the known copying apparatuses of the above described type, the development is effected either by cascading over the electrostatic latent image, the carrier particles and toner particles electrostatically attracted to each other by triboelectrical charging arising from mixing and stirring of said two particles, or by rubbing against the electrostatic latent image, the carrier particles and toner particles arranged in the form of magnetic brush through magnetic force. In the above case, however, although the toner particles in the developing material adhere to the image-formed portions by the electrostatic force of the latent image so as to be consumed thereby, the carrier particles are repeatedly used as they are without being consumed, and thus, when the developing material is used for a long period, part of the toner which does not directly contribute to the developing or the so-called "spent" toner tends to be undesirably fused over the surfaces of the carrier particles, with consequent reduction in the performance of the carrier particles which subject the toner particles to triboelectrical charging, thus resulting in adverse effects on the image quality such as reduction in density of the developed images, generation of fogging, etc.
Accordingly, in the conventional developing materials as described above, it has been necessary to replace the used developing material with a fresh one before the above adverse effects take place, for example, after developing electrostatic latent images equivalent in areas to 15,000 sheets of A4 size copy paper in a commercially available desk top type electrophotographic copying apparatus.
In order to overcome the disadvantages as described above, there has conventionally been proposed, for example, in Japanese Patent Application No. Tokugansho 53-105214 (corresponding U.S. application No. 949,426 filed Oct. 5, 1978, Kenji TABUCHI et al.) a dual-component developing material employing carrier particles of small diameter which are prepared by bonding magnetizable fine particles with resin, instead of the carrier of iron particles. The proposed developing material as described above is advantageous in that, owing to the small diameters (normally 5.about.30 .mu.) of the carrier particles, the fusion of the "spent" toner onto the surfaces of the carrier particles does not readily occur, with a consequent marked prolongation of the life (i.e. the period after which the used developing material must be disposed of) of the developing material, but the undesirable phenomenon regarding the fusion of the "spent toner" over the carrier particle surfaces still can not be avoided, and the developing material has drawbacks similar to those in the conventional dual-component developing materials in that it must undesirably be disposed of upon starting of fusion of the "spent" toner.
As a result of various studies made by the present inventors to prevent the fusion of the "spent" toner onto the surfaces of the carrier particles, it has been found that addition of electrically insulative fine particles or powder, for example, metallic oxides such as silica, alumina and the like into the developing material as a third component is effective for this purpose. Although the addition of the electrically insulative fine particles as described above is very effective for preventing the fusion of the "spent" toner to achieve long life of the developing material, there still arises a phenomenon similar to the fusion of the "spent" toner onto the surfaces of the carrier particles due to adhesion of the electrically insulative fine particles onto said carrier particle surfaces through triboelectrical charging therebetween, thus resulting in reduction of the life of the developing material by the deterioration in the performance of the carrier, contrary to the purpose of the addition in some cases.